Open the sweetened condensed milk and stir in the salt, if you want a salted caramel flavor. Pour the milk into a mason jar and top with a lid. Screw the band on the jar enough that it won't fall off, but not finger tight.
Fill the crockpot with water to just below the band. Set the temperature to LOW and cover with lid.
Let it cook for 6-8 hours, then check the color of the jar. (The lid on my crockpot is clear and I was able to watch it darken through the day.)
After about 6 hours, the milk will have turned golden brown, thick and creamy. As you continue to cook it, the dulce de leche will darken and become thicker. I wanted mine to be pourable when warm, so 6-7 hours was perfect.
Remove the jar from the crockpot and set aside to cool. Let it cool at least 1 hour before opening the jar. (I've heard that the hot dulce de leche will explode out at you if you open it too hot!) Let cool and store in the refrigerator until ready to use.
The sauce will thicken when refrigerated and can easily be reheated in the microwave for 20-30 seconds at a time, stirring in between. Enjoy!